U.S. patent number 6,102,092 [Application Number 09/098,395] was granted by the patent office on 2000-08-15 for tire having sacrificial bridging.
This patent grant is currently assigned to Michelin Recherche et Technique S.A.. Invention is credited to Robert Ciprian Radulescu.
United States Patent |
6,102,092 |
Radulescu |
August 15, 2000 |
Tire having sacrificial bridging
Abstract
The present invention provides a pneumatic tire having a tread
portion comprising a plurality of axially spaced apart essentially
longitudinal grooves separating essentially longitudinal ribs. On
at least one of said ribs, transverse grooves or cuts repeat in the
circumferential direction to form first and second land portions
wherein the first land portions comprise blocks having a
circumferential length greater than that of the second land
portions. Said second land portion acts as a sacrificial bridge
which provides traction improvement and minimizes undesirable
surface anomalies during the service life of the tire.
Inventors: |
Radulescu; Robert Ciprian
(Simpsonville, SC) |
Assignee: |
Michelin Recherche et Technique
S.A. (CH)
|
Family
ID: |
22269097 |
Appl.
No.: |
09/098,395 |
Filed: |
June 17, 1998 |
Current U.S.
Class: |
152/209.19;
152/209.23; 152/901; 152/DIG.3 |
Current CPC
Class: |
B60C
11/13 (20130101); B60C 11/0309 (20130101); B60C
11/1369 (20130101); B60C 11/12 (20130101); B60C
11/124 (20130101); Y10S 152/03 (20130101); Y10S
152/901 (20130101) |
Current International
Class: |
B60C
11/12 (20060101); B60C 11/13 (20060101); B60C
011/04 (); B60C 011/13 (); B60C 105/00 () |
Field of
Search: |
;152/209.2,209.18,209.19,209.22,209.23,DIG.3,901 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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397639 |
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Nov 1990 |
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EP |
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2-114006 |
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Apr 1990 |
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JP |
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92-310 109 |
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Dec 1990 |
|
JP |
|
2-293204 |
|
Dec 1990 |
|
JP |
|
3-153404 |
|
Jul 1991 |
|
JP |
|
5-338418 |
|
Dec 1993 |
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JP |
|
5-345505 |
|
Dec 1993 |
|
JP |
|
474588 |
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Apr 1937 |
|
GB |
|
2053783A |
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Feb 1981 |
|
GB |
|
2093777 |
|
Sep 1982 |
|
GB |
|
Primary Examiner: Maki; Steven D.
Attorney, Agent or Firm: Remick; E. Martin Reed; Robert R.
Farrell; Martin
Claims
What is claimed:
1. A radial pneumatic vehicle tire having a tread portion
comprising:
(a) a plurality of axially spaced apart essentially circumferential
grooves having a depth h in the tread portion of the tire, and
(b) at least one rib formed by the land portion between two of said
circumferential grooves, and
(c) a plurality of narrow transverse grooves having a depth h.sub.1
not exceeding the depth h of said circumferential grooves and
arranged at circumferential intervals on a at least one of said
ribs, wherein adjacent pairs of said transverse grooves define
first land portions having a circumferential length L.sub.1
circumferentially adjacent to second land portions having a
circumferential length L.sub.2, the ratio of the length L.sub.2 of
said second land portion to the length L.sub.1 of said first land
portion is such that 0.25.ltorsim.L.sub.2 /L.sub.1 .ltorsim.0.50,
and
(d) said second land portion is offset radially inward from said
first land portion a distance d.
2. The tire according to claim 1, wherein the length L.sub.1 of
said first land portion is approximately 1.0% to 1.4% of the tire
circumference.
3. The tire according to claim 1, wherein the ratio of the depth of
said narrow transverse grooves to the depth h of said
circumferential grooves is between approximately 0.75 and 1.00.
4. The tire according to claim 1, wherein the width of said narrow
grooves is about 0.2 mm to about 2.0 mm.
5. The tire according to claim 4, wherein the width of said narrow
grooves is at about 0.5 mm to about 1.0 mm.
6. The tire according to claim 5, wherein the width of said narrow
grooves is about 0.5 mm.
7. The tire according to claim 1, wherein the ratio of said
recessed distance d to the depth h of said circumferential grooves
is 0.10.ltorsim.d/h.ltorsim.0.20.
8. The tire according to claim 1, wherein said recessed distance d
is about 2 mm to about 4 mm.
9. The tire according to claim 1, wherein the ratio of the length
L.sub.2 of said second land portion to the length L.sub.1 of said
first land portion is such that 0.40.ltorsim.L.sub.2 /L.sub.1
.ltorsim.0.50.
10. The tire according to claim 1, wherein the length L.sub.2 of
said second land portion is about 16 mm.
11. A radial pneumatic vehicle tire having a tread portion
comprising:
(a) a plurality of axially spaced apart essentially circumferential
grooves having a depth h in the tread portion of the tire, and
(b) at least one rib formed by the land portion between two of said
circumferential grooves, and
(c) a plurality of narrow transverse grooves having a depth h.sub.1
not exceeding the depth h of said circumferential grooves and
arranged at circumferential intervals on a at least one of said
ribs, wherein alternating pairs of said transverse grooves define
first land portions having a circumferential length L.sub.1
circumferentially adjacent to second land portions having a
circumferential length L.sub.2, the ratio of the length L.sub.2 of
said second land portion to the length L.sub.1 of said first land
portion is such that 0.25.ltorsim.L.sub.2 /L.sub.1 .ltorsim.0.50,
and
(d) said second land portion is offset radially inward from said
first land portion a distance d, and
(e) at least one of said narrow grooves has an angle of inclination
.alpha. relative to the radially outward direction.
12. The tire according to claim 11, wherein a first narrow groove
disposed at the leading edge of a sacrificial bridge has an angle
of inclination relative to the radially outward direction of about
-5.degree. to about -15.degree..
13. The tire according to claim 11, wherein a second narrow groove
disposed at a trailing edge of a sacrificial bridge has an angle of
inclination relative to the radially outward direction of about
-5.degree. to about -15.degree..
14. The tire according to claim 11, wherein a first narrow groove
disposed at the leading edge of a sacrificial bridge has an angle
of inclination relative to the radially outward direction of about
-5.degree. to about -15.degree., and wherein a second narrow groove
disposed at a trailing edge of said sacrificial bridge 30 has an
angle of inclination relative to the radially outward direction of
about -5.degree. to about -15.degree..
15. The tire according to claim 14 wherein both first and second
narrow grooves disposed respectively at the leading edge and the
trailing edge of said sacrificial bridge have an angle of
inclination relative to the radially outward direction of about
-10.degree..
16. The tire according to claim 11, wherein the length L.sub.1 of
said first land portion is approximately 1.0% to 1.4% of the tire
circumference.
17. The tire according to claim 11, wherein the ratio of the depth
of said narrow transverse grooves to the depth h of said
circumferential grooves is about 0.75 to about 1.00.
18. The tire according to claim 11, wherein the width of said
narrow grooves is about 0.2 mm to about 2.0 mm.
19. The tire according to claim 18, wherein the width of said
narrow grooves is about 0.5 mm to about 1.0 mm.
20. The tire according to claim 19, wherein the width of said
narrow grooves is about 0.5 mm.
21. The tire according to claim 11, wherein the ratio of said
recessed distance d to the depth h of said circumferential grooves
is 0.10.ltorsim.d/h.ltorsim.0.20.
22. The tire according to claim 11, wherein said recessed distance
d is between about 2 mm to about 4 mm.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radial pneumatic vehicle tire
for which tread surface anomalies causing user dissatisfaction are
diminished without decrease in tire performance such as wet
traction and braking performance. More specifically, the invention
relates to a pneumatic tire having a plurality of axially spaced
apart essentially longitudinal grooves separating essentially
longitudinal ribs. On at least one of said ribs, transverse grooves
or cuts repeat in the circumferential direction to form first and
second land portions wherein the first land portions comprise
blocks having a circumferential length greater than that of the
second land portions.
2. Description of Related Art
In order to improve the wet traction, wet grip, braking performance
and the like, radial pneumatic tires have treads with longitudinal
or zigzag grooves extending in the circumferential direction, and,
for further traction improvement, lateral grooves axially
connecting the circumferential grooves to form blocks. To maintain
a good level of traction performance, the lateral grooves or cuts
need to be present throughout the service life of the tire tread.
Unfortunately, to achieve this the tire must have lateral grooves
whose depth is substantially equal to the depth of the longitudinal
grooves. An example of such a prior art tire 100 is shown in FIGS.
1a and 1b, respectively, in a full tire view and a plan view of the
tread portion of the tire. In this example the tread blocks 20 are
circumferentially spaced apart by the substantially full depth
lateral grooves 30. Tire treads so designed are commonly used on
the drive axle of vehicles and have acceptable wet traction
performance, but are known to have reduced tread rigidity resulting
in the formation of tread surface anomalies such as a
"heel-and-toe" or "sawtooth" profile or tread block depression.
These anomalies result in user dissatisfaction due to either
unacceptable visual appearance of the tire or ride discomfort
caused by tread induced vibrations. Either factor can cause removal
of the tire from service prior to delivering its full potential
tread service to the user.
To achieve some kind of compromise between surface anomalies and
traction performance, tires have been designed having lateral
grooves defining blocks 20 where the lateral grooves 30 have a
depth d substantially less than the depth h of the longitudinal
grooves, an example of which is tire 200 shown in FIG. 2a. The land
portions of the tread bounded by edge 22 of a first block 20 and by
edge 21 of a second block 20 are commonly referred to as "bridges".
For values of d/h near zero, tires will have poor traction, and for
values of d/h approaching unity, tires may develop surface
anomalies leading to reduced service life of the original tread. An
acceptable result can be obtained when the tire tread is designed
so that the ratio R.sub.1 =d/h of groove depth d to the tread depth
h is such that d/h is between about 0.1 to about 0.2.
Unfortunately, tires experience a loss of tread rubber due to
factors such as abrasion, fatigue and the like during their service
lives. As a result, tires having tread designs such as shown in
FIG. 2a, that is with shallow transverse grooves, will wear in such
a manner that the ratio d/h will continually decrease and
eventually approach a value of zero. The disadvantage of such a
tire wherein d/h approaches zero is the aforementioned loss of wet
grip, braking performance and the like.
Tests under highway use conditions were conducted on tires such as
tire 200 having a new tire tread depth of approximately 20.5 mm
with lateral
grooves approximately 3 mm deep. The evolution of d/h just
described is demonstrated by the test results shown in FIG. 2b
which shows the measured tread depth versus circumferential
position for a section of the tire. After 54,000 kilometers of
service the tread depth has reached an approximate value of 17 mm
everywhere, and the ratio of d/h is approximately zero. In this
case the tires are more often removed from service for a perceived
loss of traction rather than for the onset of surface anomalies. In
an effort to mitigate this counterperformance, tire designers often
add additional siping or employ complex block geometry which,
instead of improving the situation, may further generate surface
anomalies and/or sensitivity to chipping or tearing. Thus a tire
tread design that maintains the optimum value of the ratio d/h
throughout the service life of the tread is needed.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved radial
pneumatic tire which maintains good wet traction performance and is
free of surface anomalies. This object is obtained by a tread
portion of the tire having a plurality of longitudinal grooves
which form ribs and at least one of those ribs being transected by
narrow transverse groove or cuts which form alternating land
portions wherein the first land portions are longer than the second
land portions. According to the notation shown in FIG. 3b for tire
300, the first land portion will hereafter be referred to as block
20 and the second land portion as sacrificial bridge 30. An object
of the invention is to maintain a non-zero value of the ratio
R.sub.1 =d/h. To accomplish this object, the sacrificial bridge
must be decoupled from the adjacent tread blocks 20. This
decoupling can be achieved by narrow transverse grooves or cuts 40
and 50. Cut 40 is located between the trailing edge 22 of a first
of blocks 20 and a leading edge 31 of the sacrificial bridge 30.
Cut 50 is located between the trailing edge 32 of the sacrificial
bridge 30 and the leading edge 21 of a second of blocks 20. Leading
and trailing edges are defined relative to the rolling direction of
the tire with the leading edge 21 being the first point on block 20
to engage the ground during rolling of the tire and the trailing
edge 22 being the last point on block 20 to engage the ground
during rolling of the tire. The sacrificial bridge 30 is bounded in
its circumferential extent by cuts 40 and 50 and in its lateral
extent by circumferential grooves 10. The depth h.sub.1 of the cuts
40 and 50 and the height h.sub.2 of the sacrificial bridge 30 are
such that the surface 33 of the sacrificial bridge 30 contacts the
ground during rolling of the tire under load. An example of such a
design is the tire 300 shown in plan view in FIG. 3a.
Since the surface 33 contacts the ground when rolling under load,
the sacrificial bridge 30 will be subjected to longitudinal
shearing forces during the period of ground contact. This shearing
force must be sufficient to generate a rate of rubber loss
(measured in mm/km) from the sacrificial bridge such that the ratio
d/h is maintained. In order to solve the problems found in prior
art tires, the inventor has found that an optimum level of shearing
force, and thus, rate of rubber loss will be obtained only for
certain ranges of the values of R.sub.1 =d/h, R.sub.2 =h.sub.2
/h.sub.1, and the ratio of sacrificial bridge length L.sub.2 to
block length L.sub.1, R.sub.3 =L.sub.2 /L.sub.1. Only when these
parameters are in their respective optimum ranges will the ratio
d/h be maintained throughout the service life of the tread.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and embodiments will be described with reference to
the accompanying drawings, wherein:
FIG. 1a is a partial perspective view of a pneumatic radial tire
100 corresponding to the prior art having full depth transverse
grooves.
FIG. 1b is a plan view of the tread portion of a pneumatic radial
tire 100 corresponding to the prior art shown in FIG. 1a.
FIG. 2a is a plan view of the tread portion of a pneumatic radial
tire 200 corresponding to the prior art having partial depth
transverse grooves.
FIG. 2b is a cross sectional view of the tread portion of a
pneumatic radial tire 200 corresponding to the prior art shown in
FIG. 2a.
FIG. 2c is a graphical plot of tread depth vs. circumferential
position around a tire having the tread portion shown in FIG.
2a.
FIG. 3a is a plan view of the tread portion of a pneumatic radial
tire 300 corresponding to a first embodiment of the invention.
FIG. 3b is a cross-sectional view taken along the midline of the
tread portion shown in FIG. 3a wherein the groove edge sipes have
been removed for clarity.
FIG. 3c is a graphical plot of the tread depth vs. circumferential
position around a tire having the tread portion shown in FIG. 3a.
Note: Direction of tire rotation indicated by the uppercase R.
FIG. 4a and FIG. 4b are cross-sectional views of the tread portion
of a pneumatic radial tire 300 corresponding to a second embodiment
of the invention showing possible configurations for inclined cuts.
Note: Direction of tire rotation indicated by the uppercase R.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
When a block type tire is operated on a vehicle during highway use,
surface shearing stresses are developed at the tire-road interface
due the flattening of the tire carcass and belt structure and due
to compression of the tread block elements. Since the tread surface
is disposed at a radially outward position greater than that of the
belt structure, rolling into contact with a flat surface causes a
tangential stress to develop at the tire road interface in an
advancing direction during approximately the first half of contact
and in a retarding direction during approximately the second half
of contact. For tires having block or block-type tread designs, a
second set of stresses is generated due to the vertical compressive
strain of the tread rubber induced by the vertical load applied to
the inflated tire. This second set of stresses acts in an advancing
sense at the transverse edge of the block first contacting the
ground and in a retarding sense at the transverse edge of the block
last contacting the ground. These transverse edges are referred to
respectively as the leading edge 21 and trailing edge 22 of the
block 20 shown in FIG. 3b. The two sets of stresses act
simultaneously on the block surface with a resultant rate of tread
rubber loss which can be non-uniform across the block surface.
Specifically the rate of tread rubber loss is often a maximum at or
near the trailing edge 22 of the block. The tread surface profile
resulting from this non-uniform tread rubber loss is commonly known
as a "heel-and-toe" or "sawtooth" profile. In a later stage, such a
tread surface profile can result in the rapid depression of some
tread blocks relative to adjacent blocks and may necessitate
premature removal of the tire from service.
In the present invention a sacrificial bridge is provided between
tread blocks. The presence of the sacrificial bridge minimizes the
undesirable "heel-and-toe" or "sawtooth" profile while at the same
time maintaining acceptable traction performance. To achieve the
above object according to the present invention, a radial pneumatic
radial tire has a tread portion comprising: (a) a plurality of
axially spaced apart essentially circumferential grooves having a
depth h in the tread portion of the tire, and (b) at least one rib
formed on the land portion between two of said circumferential
grooves, and (c) a plurality of transverse grooves having a depth
h.sub.1 not exceeding the depth h of said circumferential grooves
and arranged at circumferential intervals on at least one of the
ribs, wherein alternating pairs of said transverse grooves define
first land portions having a circumferential length L.sub.1
circumferentially adjacent to second land portions having a
circumferential length L.sub.2, the ratio of the length L.sub.2 of
said second land portion to the length L.sub.1 of said first land
portion is such that 0.25.ltorsim.L.sub.2 /L.sub.1 .ltorsim.0.50,
and (d) said second land portion is offset radially inward from
said first land portion a distance d. By the proper specification
of the ratio of the length of the sacrificial bridge to the length
of the tread block and the depth and width of the cuts at the
leading and trailing edges of the sacrificial bridge, the desired
effect is maintained throughout the service life of the tread.
FIGS. 3a and 3b, 4a and 4b show various embodiments of the tire
according to the present invention. In these embodiments a
plurality of circumferential grooves 10 are arranged at regular
axial intervals across the tread portion of the tire. The number
and specific axial position of the circumferential grooves is
determined according to the intended application of the tire.
Circumferential grooves 10 have a depth h in the radial direction.
A plurality of ribs is formed between adjacent circumferential
grooves. In these embodiments, the ribs are divided in the
transverse direction by a plurality of circumferentially spaced
cuts 40 and 50. A first land portion, block 20, has a surface at
the most radially outward position of the tread portion of the
tire. Block 20 has a length L.sub.1 in the circumferential
direction, and a height h in the radial direction equal to the
depth of the circumferential grooves 10. A second land portion,
sacrificial bridge 30, has a surface 33 spaced radially inward from
the surface of block 20 by a distance d. Surface 33 of sacrificial
bridge 30 has a length L.sub.2 measured in the circumferential
direction. Cuts 40 and 50 have a depths h.sub.1 measured radially
inward from the tread surface and widths w.sub.1 and w.sub.2,
respectively, measured in the tire circumferential direction. Cuts
40 and 50 are shown in the figures as straight vertical cuts having
equal depths h.sub.1 although the invention encompasses cuts 40 and
50 having unequal depths or differing alternative shapes.
When a tire according to the present invention is mounted on a rim,
inflated and loaded according to recommendations of the Tire and
Rim Association, rolling the tire against the ground causes the
above-mentioned sheer stresses to be generated on blocks 20. When
sacrificial bridges 30 are present, compression of the tread rubber
causes the vertical walls of cuts 40 and 50 to approach each other
so that sacrificial bridge 30 now acts to buttress the adjacent
tread blocks 30 against the action of the aforementioned shear
forces and thereby improve the uniformity of tread rubber loss
across the surface of the block. Land portion 33 of sacrificial
bridge 30 is also subjected to similar stress mechanisms. Due to
the presence of cuts 40 and 50, the sacrificial bridge is free to
undergo shear deformation and rubber loss such that the depression
d is maintained. If the specific dimensions of sacrificial bridge
30, are such that the land portion has insufficient resistance to
shear deformation, then the rate of rubber loss from bridge surface
33 will be less than the rate of rubber loss form block portion 20
In this case, depression d disappears after a low number of service
miles and the ratio d/h approaches zero.
FIG. 3a shows a first embodiment of the tire according to the
invention. In this embodiment, blocks 20 are formed in the ribs
between circumferential grooves with sacrificial bridges 30 being
formed between the blocks 20 by vertical narrow cuts 40 and 50.
Sacrificial bridges 30 are located at regular circumferential
intervals around the circumference of the tire. In FIG. 3a, blocks
20 are shown with a uniform length L. Typically length L.sub.1 is
between approximately 1.0% to 1.4% of the tire circumference
although L.sub.1 may have multiple discrete values so as to create
a sequence of discrete block pitch lengths. Within the teachings of
the invention, both the actual values of L.sub.1 and the sequence
of the discrete pitches are typically determined to minimize
undesirable tire noise. Sacrificial bridges 30 are shown with
straight edges 31 and 32 having an intersection angle .beta.
relative to the tire rolling direction as shown in FIG. 3a. Angle
.beta., is preferably in a range of about 60.degree. to about
90.degree.. The invention encompasses edges 31 and 32 which may
take on alternative zigzag or curvilinear shapes.
EXAMPLE CASE
The invention disclosed herein can be advantageous for all classes
of pneumatic tires where there is a need to improve the compromise
between traction capabilities and overall service life. In order to
demonstrate the improvements possible with the present invention,
three different designs according to this first embodiment were
prepared on 275/80 R 22.5 heavy duty truck radial tires and then
were mounted on the drive axles of 6.times.4 heavy duty trucks
operated under highway service conditions. Each design was mounted
with a companion set of prior art tires. During the course of the
test, tread depths and tread surface profiles were measured as well
as notations of the appearance of tread surface anomalies. The
specifics of the three designs and the prior art reference tire are
shown in Table 1.
From the results of these tests, a tire according to the invention
described by this embodiment could maintain an acceptable recess
and thus d/h for up to 144,000 km (90,000 miles). The results shown
in Table 1 clearly demonstrate that only certain combinations of
the design parameters yield this level of performance. Between
Embodiment 1-1 and Embodiment 1-2, all parameters have been held
constant except the initial depression d. Nevertheless, the mileage
to d/h.about.0 is essentially equivalent at 80,000 km (50,000
miles). Embodiment 1-3 exhibits superior performance of 144,000 km
(90,000 miles). In this instance, the ratio R.sub.3 =L.sub.2
/L.sub.1 has a value of 0.42.
TABLE 1 ______________________________________ Example Cases Using
Embodiment 1 Design Tread Design Parameters Prior Art Embodiment
Embodiment Embodiment See FIG. 3b (Reference) 1-1 1-2 1-3
______________________________________ Block 45 42 42 38 Length
L.sub.1 (mm) Bridge 8 12 12 16 Length L.sub.2 (mm) Width of N/A 0.5
0.5 0.5 First Cut w.sub.1 (mm) Width of N/A 0.5 0.5 0.5 Second Cut
w.sub.2 (mm) Initial 3 2 3 3 Depression d (mm) Tread Depth 20 20 20
20 h (mm) Cut Depth N/A 20 20 20 h.sub.1 (mm) Bridge N/A 18 17 17
Height h.sub.2 (mm) R.sub.1 = d/h 0.15 0.10 0.15 0.15 R.sub.2 =
h.sub.1 /h N/A 1.00 1.00 1.00 R.sub.3 = L.sub.2 /L.sub.1 0.18 0.29
0.29 0.42 Distance to 54,000 80,000 80,000 144,000 d/h .about. 0
(km) ______________________________________
The data show that an effective range for R.sub.3 is
0.25.ltorsim.L.sub.2 /L.sub.1 .ltorsim.0.50, and preferably R.sub.3
should have a value greater than about 0.40 up to about 0.50.
Values of R.sub.3 less than 0.25 will yield a sacrificial bridge
whose rate of rubber loss will be insufficient to show substantial
improvement in maintaining the depression d. Conversely values of
R.sub.3 greater than 0.50 mean that the total surface
area of blocks 20 would be insufficient to provide adequate
tractive forces or would produce an accelerated rate of tread
rubber loss.
The ratio R.sub.1 =d/h varies in the test cases between 0.10 to
0.15, and an effective range has been found to be
0.10.ltorsim.R.sub.1 .ltorsim.0.20. Preferably R.sub.1 is
approximately 0.15. In all examples shown in Table 1 the depth of
cuts 40 and 50 is equal to the tread depth, h, or, alternatively
R.sub.2 =1.00. However, habitual practice by users of heavy duty
truck tires often leads to removal of a tire from service with some
tread remaining. This allows cuts 40 and 50 to be less deep than
the tread depth h, but, in all cases, maintenance of acceptable wet
traction performance during actual service requires h.sub.1 to be
at least 75% of the tread depth h. This leads to a specification of
0.75.ltorsim.R.sub.2 .ltorsim.1.00 and, preferably that R.sub.2 is
approximately 1.00. The widths of cuts 40 and 50 are the same and
equal to 0.5 mm in this embodiment. Widths w.sub.1 and w.sub.2 are
effective in the range of about 0.2 mm to about 2.0 mm. Preferably,
widths w.sub.1 and w.sub.2 are between about 0.5 mm to about 1.0
mm. Unfortunately, concentrated stresses at the bottom of narrow
cuts 40 and 50 can produce cracking which can cause the early
removal of the tire from service. To reduce this stress
concentration, cuts 40 and 50 require a minimum radius at the
bottom of the groove of about 1.0 mm. As a means to reduce this
stress concentration, cuts 40 and 50 as well as the groove edge
siping are shown in the figures with an enlarged portion at their
most radially inward extent.
Results from vehicle tests such as those shown in Table 1 and in
FIG. 3c indicate that the depression d is maintained well adjacent
to the leading edge of block 30 but is considerably diminished
adjacent to the trailing edge. In spite of the improved performance
obtained by the presence of the sacrificial bridge, the tread
rubber loss profile of block 20 exhibits a tendency for sawtooth
shape. A way to obtain the desired improvement of a more uniform
height of the block 20 throughout the service life of the tread, is
to incline at least one of the cuts 40 or 50 relative to the
outward normal from the tread surface.
A second embodiment of the invention is shown in FIGS. 4a and 4b
wherein cuts 40 and 50 may be inclined with respect to the outward
normal from the tread surface. In these cases the axes of the cuts
40 and 50 have inclination angles .alpha..sub.1 and .alpha..sub.2,
respectively, relative to the outward normal from the tread
surface. Angle .alpha. is positive when the groove axis is rotated
in the direction of tire rotation or counterclockwise as shown in
FIGS. 4a and 4b. In the first example shown in FIG. 4a only cut 40
is inclined in the range -15.degree..ltorsim..alpha..sub.1
.ltorsim.-5 and preferably .alpha..sub.1 is approximately
-10.degree.. In another example (not shown) only cut 50 is inclined
in the range -15.degree..ltorsim..alpha..sub.2 .ltorsim.-5 and
preferably .alpha..sub.2 is approximately -10.degree.. For the
example shown in FIG. 4b, both cuts 40 and 50 are inclined and
.alpha..sub.1 and .alpha..sub.2 have negative values. In the
example of FIG. 4b, inclination of the grooves can be effective
over a range -15.degree..ltorsim..alpha..sub.1 or .alpha..sub.2
.ltorsim.-5.degree. and preferably .alpha..sub.1 and .alpha..sub.2
are both approximately -10.degree.. Inclination of cuts 40 and/or
50 causes the tire to be directional, that is, having a preferred
direction of rotation. This is shown in FIGS. 4a and 4b by the
uppercase R. It is also customary for this preferred direction of
rotation to be indicated on the tire by an arrow or an
advisory.
* * * * *